Continuous method for separating a C4 cut

ABSTRACT

In a continuous process for fractionating a C 4  fraction (C 4 ) by extractive distillation using a selective solvent (LM) in an extractive distillation column (EDK), it is proposed that a dividing wall (TW) is installed in the longitudinal direction in the extractive distillation column (EDK) to form a first region (A), a second region (B) and a lower combined column region (C) and a top stream (C 4 H 10 ) comprising the butanes is taken off from the first region (A), a top stream (C 4 H 8 ) comprising the butenes is taken off from the second region (B) and a stream (C 4 H 6 ) comprising the hydrocarbons from the C 4  fraction which are more soluble in the selective solvent (LM) than are the butanes and the butenes is taken off from the lower combined column region (C).

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a national stage application of International PatentApplication No. PCT/EP03/07991 filed on Jul. 22, 2003, and claimspriority to German Patent Application No. 102 33 620.2 filed on Jul. 24,2002, both of which are incorporated herein by reference in theirentireties.

The present invention relates to a continuous process for fractionatinga C₄ fraction by extractive distillation using a selective solvent andto an extractive distillation column suitable for this purpose.

The term C₄ fraction refers to mixtures of hydrocarbons havingpredominantly four carbon atoms per molecule. C₄ fractions are obtained,for example, in the production of ethylene and/or propylene by thermalcracking, usually in steam crackers or FCC (Fluid Catalytic Cracking)plants, of a petroleum fraction such as liquefied petroleum gas,naphtha, or gas oil. C₄ fractions are also obtained in the catalyticdehydrogenation of n-butane and/or n-butene. C₄ fractions generallycomprise butanes, n-butene, isobutene, 1,3-butadiene, together withsmall amounts of other hydrocarbons including butynes, in particular1-butyne (ethylacetylene) and butenyne (vinylacetylene). The1,3-butadiene content of C₄ fractions from steam crackers is generallyfrom 10-80% by weight, preferably from 20-70% by weight, in particularfrom 30-60% by weight, while the content of vinylacetylene andethylacetylene generally does not exceed 5% by weight.

The fractionation of C₄ fractions is a complicated distillation problembecause of the small differences in the relative volatilities of thecomponents. Fractionation is therefore carried out by extractivedistillation, i.e. a distillation with addition of a selective solvent(also referred to as extractant) which has a boiling point higher thanthat of the mixture to be fractionated and increases the differences inthe relative volatilities of the components to be separated.

Many processes are known for the fractionation of C₄ fractions by meansof extractive distillation using selective solvents. In all of them, thegaseous C₄ fraction to be fractionated is brought into countercurrentcontact with the liquid selective solvent under appropriatethermodynamic conditions, generally at low temperatures, frequently inthe range of 20-80° C., and at moderate pressures, frequently fromatmospheric pressure to 6 bar, so that the selective solvent is loadedwith the components of the C₄ fraction for which it has a relativelyhigh affinity, while the components for which the selective solvent hasa lower affinity remain in the vapor phase and are taken off at the top.The components are subsequently fractionally liberated from the ladensolvent stream in one or more further process steps under suitablethermodynamic conditions, i.e. at higher temperature and/or lowerpressure compared to the first process step.

The extractive distillation of C₄ fractions is frequently carried out insuch a way that the components of the C₄ fraction for which theselective solvent has a lower affinity than for 1,3-butadiene, inparticular the butanes and the butenes, remain essentially in the gasphase while 1,3-butadiene and further hydrocarbons for which theselective solvent has a higher affinity than for 1,3-butadiene areessentially completely absorbed by the selective solvent. The gas phaseis taken off at the top and is frequently referred to as raffinate 1.Such a process is described, for example, in DE-A 198 188 10, where theraffinate 1 is the stream denoted by Gbc taken off from the top of theextractive distillation column E I in FIGS. 1 and 2.

However, for the further use of raffinate 1, it is generally moreeconomical for the butanes and butenes to be present as separatestreams. The apparatuses used in the subsequent steps for the furtherprocessing of the butenes can be made smaller as a result and thebutanes can be obtained directly as valuable cracker feed.

DE-A 102 193 75 therefore proposes a process for fractionating a C₄fraction by extractive distillation so as to give butanes and butenes inseparate streams. However, this requires two process stages, with abutane-containing top stream being taken off from a scrubbing zone in afirst process stage I and a butene-containing top stream being taken offfrom a degassing zone in a second process stage II.

It is an object of the invention to provide an improved, in particularmore economical and less energy-intensive, process for separating a C₄fraction into 1,3-butadiene, butenes and butanes as three separatestreams by extractive distillation and to provide an extractivedistillation column suitable for this purpose. We have found that thisobject is achieved by a continuous process for fractionating a C₄fraction by extractive distillation using a selective solvent in anextractive distillation column, wherein a dividing wall is installed inthe longitudinal direction in the extractive distillation column to forma first region, a second region and a lower combined column region and atop stream comprising the butanes is taken off from the first region, atop stream comprising the butenes is taken off from the second regionand a stream comprising the hydrocarbons from the C₄ fraction which aremore soluble in the selective solvent than are the butanes and thebutenes is taken off from the lower combined column region.

The present invention thus provides a process for fractionating a C₄fraction by extractive distillation and also a suitable extractivedistillation column which make it possible for butanes and butenes to beseparated off as separate streams from the top of a single extractivedistillation column.

According to the present invention, the extractive distillation iscarried out in a dividing wall column in which a dividing wall isinstalled so as to form a first region, a second region and a lowercombined column region.

Dividing wall columns are, as is known, used for relatively complexseparation tasks, generally for mixtures of at least three componentswhich are each to be obtained in pure form. They have a dividing wall,i.e. a flat sheet which is generally aligned in the longitudinaldirection of the column and prevents crossmixing of liquid and vaporstreams in the regions of the column.

For the present purposes, use is made of a particular type of dividingwall column whose dividing wall continues through to the uppermost pointof the column and thus allows mixing of liquid and vapor streams only inthe lower combined column region. The regions referred to as the firstand second regions are separated from one another by the dividing wall.

The length of the dividing wall and also its horizontal position in theextractive distillation column can be different depending on thecomposition of the C₄ fraction fed to the extractive distillation columnand on the specifications for the fractions to be separated off by meansof the extractive distillation column. It is thus possible, for example,for the dividing wall to be located centrally or away from the center.

According to the present invention, a top stream comprising the butanesis taken off from the first region of the extractive distillation columnconfigured as a dividing wall column and a top stream comprising thebutenes is taken off from the second region. A stream *comprising thehydrocarbons from the C₄ fraction which are more soluble in theselective solvent than are the butanes and the butenes is taken off fromthe lower combined column region.

The solvents which are suitable for the present separation task are oneswhose affinity for hydrocarbons increases from hydrocarbons havingsingle bonds to hydrocarbons having double bonds and further tohydrocarbons having conjugated double bonds and triple bonds, preferablydipolar, particularly preferably dipolar aprotic, solvents. To protectthe apparatus, substances which are noncorrosive or have a lowcorrosivity are preferred.

Examples of suitable selective solvents for the process of the presentinvention are butyrolactone, nitriles such as acetonitrile,propionitrile, methoxypropionitrile, ketones such as acetone, furfural,N-alkyl-substituted lower aliphatic acid amides, such asdimethylformamide, diethylformamide, dimethylacetamide,diethylacetamide, N-formylmorpholine, N-alkyl-substituted cyclic acidamides (lactams) such as N-alkylpyrrolidones, in particularN-methylpyrrolidone, hereinafter referred to as NMP for short. Use isgenerally made of alkyl-substituted lower aliphatic acid amides orN-alkyl-substituted cyclic acid amides. Dimethylformamide, acetonitrile,furfural and in particular NMP are particularly advantageous.

However, it is also possible to use mixtures of these solvents among oneanother, for example a mixture of NMP with acetonitrile, or mixtures ofthese solvents with cosolvents such as water and/or tert-butyl ethers,for example methyl tert-butyl ether, ethyl tert-butyl ether, propyltert-butyl ether, n- or iso-butyl tert-butyl ether.

A particularly useful solvent is NMP, preferably in aqueous solution,advantageously containing from 0-20% by weight of water, in particularfrom 7-10% by weight of water, particularly preferably 8.3% by weight ofwater.

As C₄ fraction for use in the present process, it can be advantageous touse a mixture of hydrocarbons obtained by thermal cracking of apetroleum fraction. Such a mixture typically has compositions in % byweight in the following ranges:

1,3-butadiene from 10 to 80 butenes from 10 to 60 butanes from 5 to 40other C₄-hydrocarbons from 0.1 to 5 and other hydrocarbons, inparticular C₃- and C₅-hydrocarbons from 0 to not more than 5.

However, the invention is not restricted in respect of the C₄ fractionswhich can be used. For example, it is also possible to use C₄ fractionsfrom FCC (Fluid Catalytic Cracking) plants, which generally comprisefrom 20-70% by weight of butanes, from 30-80% by weight of butenes andother C₃-C₅-hydrocarbons as balance.

The C₄ fraction is fed in gaseous or liquid form into the extractivedistillation column, preferably into the first region which ispartitioned off by means of the dividing wall. The C₄ fraction isparticularly preferably fed in at about the middle of the first regionof the extractive distillation column.

The selective solvent is introduced as a liquid stream into both regionsof the extractive distillation column, in each case in the upper part ofthe region.

Preference is given to taking off a stream comprising the butanes invapor form as top stream from the first region of the extractivedistillation column, condensing it in a condenser at the top of thecolumn, returning part of the condensate as runback to the first regionand taking off the remainder.

Correspondingly, a top stream comprising the butenes is taken off fromthe second region of the extractive distillation column and ispreferably condensed in a condenser, part of the condensate is returnedas runback to the second region and the remainder is taken off.

The form of words used above stating that the top stream from the regionof the extractive distillation column in each case comprises butanes orbutenes means that the respective streams comprise, depending on therequired specification, predominantly butanes or butenes, i.e. generallyat least 80% by weight of butanes or butenes, preferably from 95-99% byweight of butanes or butenes. In specific cases, specifications havingpurities of above 99% by weight of butanes or butenes may also berequired. The further components of the respective top streams are, inparticular, butanes in the butene stream and vice versa and also tracesof further hydrocarbons.

According to the present invention, a stream comprising the hydrocarbonsfrom the C₄ fraction which are more soluble in the selective solventthan are the butanes and the butenes is taken off from the lowercombined column region of the extractive distillation column.

Preference is given to taking off the stream comprising the hydrocarbonsfrom the C₄ fraction which are more soluble in the selective solventthan are the butanes and the butenes as side stream from the lowercombined column region and taking off the selective solvent as bottomstream. In this embodiment, a desorption step for separating off thehydrocarbons from the solvent laden therewith is thus integrated intothe lower part of the lower combined region of the extractivedistillation column. However, it is likewise possible to carry out thedesorption step in an apparatus separate from the extractivedistillation column, i.e. firstly taking off the hydrocarbons from theC₄ fraction which are more soluble in the selective solvent than are thebutanes and the butenes together with the selective solvent as bottomstream from the extractive distillation column.

A dividing wall column as extractive distillation column is equivalentto an assembly of two or more, in particular two or three, thermallycoupled columns. The energy requirement here is comparable to that forthe corresponding dividing wall column. The invention thus alsoencompasses all variants of the apparatus in which the extractivedistillation column is configured not as a dividing wall column but astwo or more, in particular two or three, thermally coupled columns.

The invention is not limited in terms of the separation-active internalswhich can be used in the extractive distillation column.

The number of theoretical plates in the region of the dividing wall isdependent, in particular, on the composition of the C₄ fraction fed in,on the solvent used and on the required specifications for the topstream comprising the butanes and that comprising the butenes.Preference is given to from 10-80 theoretical plates, in particular 25theoretical plates, being located in the region of the dividing wall inthe extractive distillation column.

The composition of the C₄ fraction fed in, the solvent used and therequired specifications for the top stream comprising the butanes andthat comprising the butenes are also critical factors in the selectionof the feed tray for the C₄ fraction, preferably in the first region ofthe extractive distillation column, and for the inflow rates of theselective solvent introduced in the upper part of the first and secondregions.

A condenser for the vapor streams is advantageously provided in each ofthe regions at the top of the extractive distillation column.

In addition, it is advantageous to carry out a heterogeneously catalyzedselective hydrogenation of the hydrocarbons containing triple bonds fromthe C₄ fraction to hydrocarbons containing double bonds by means ofhydrogen in the extractive distillation column. This requires theinstallation of suitable internals provided with heterogeneous catalystsin the extractive distillation column and the introduction of a streamof hydrogen into the column, preferably below the inlet for the C₄fraction, in the lower combined region of the extractive distillationcolumn.

The stream comprising the butenes, i.e. 1-butene, 2-butenes (cis andtrans) and isobutene, obtained in the present process can be processedfurther in various ways.

The term “comprising predominantly” used in the following description inthe context of identification of streams means that the streams containat least 60% by weight, preferably at least 80% by weight, particularlypreferably at least 95% by weight, of the main component indicated ineach case.

The further processing can advantageously be carried out to giveisobutene as a single desired product or additionally to give a productcomprising 1-butene or a product comprising 2-butenes.

In a first process variant, the further processing can be carried out ina reactive distillation column to give a stream comprising predominantlyisobutene and a stream comprising predominantly 2-butenes, with the1-butene being hydroisomerized to 2-butenes in the reactive distillationcolumn and the stream comprising predominantly isobutene being taken offas top stream from the reactive distillation column and the streamcomprising predominantly 2-butenes being taken off as bottom stream fromthe reactive distillation column.

In a further process variant, the stream comprising the butenes issubjected to a selective etherification of the isobutene and separationinto a stream comprising the isobutene ether and a stream comprising1-butene and 2-butenes, and the stream comprising 1-butene and the2-butenes is subsequently processed further by gas-phase isomerizationof the 2-butenes to give a stream comprising predominantly 1-butene orby hydroisomerization of the 1-butene to give a stream comprisingpredominantly 2-butenes.

It is also possible to carry out the further processing by skeletalisomerization of 1-butenes and 2-butenes to isobutenes, giving a streamcomprising predominantly isobutene.

The further processing of the butenes-containing stream obtained by thepresent process can also be carried out so as to give no isobutene asdesired product, but instead be carried out according to one of thefollowing process variants:

In one process variant, isobutene is separated off and worked up byskeletal isomerization to give a stream comprising predominantly1-butene and 2-butenes.

In a further variant, isobutene is separated off and processed furtherby hydrogenation to give a stream comprising predominantly isobutanewhich is preferably fed to a cracker or is processed further by skeletalisomerization to give a stream comprising predominantly n-butane anddehydrogenation of the latter to give a stream comprising predominantly1-butene and 2-butenes.

It is also possible for the isobutene in the stream comprising thebutenes to be selectively dimerized to the corresponding C₈hydrocarbons.The C₈hydrocarbons can subsequently be separated off from a streamcomprising 1-butene and 2-butenes in a simple distillation.

In a preferred process variant, the stream comprising the hydrocarbonswhich are more soluble in the selective solvent than are the butanes andthe butenes which is taken off from the extractive distillation columnis worked up further by distillation. Here, the stream comprising thehydrocarbons which are more soluble in the selective solvent than arethe butanes and butenes which is taken off from the extractivedistillation column is fed to a first distillation column in which it isseparated into a top stream comprising 1,3-butadiene, propyne, possiblyfurther low boilers and possibly water, and a bottom stream comprising1,3-butadiene, 1,2-butadiene, acetylenes and possibly further highboilers, with the proportion of 1,3-butadiene in the bottom stream fromthe distillation column being regulated so that it is sufficiently highto dilute the acetylenes to outside the range in which there is a riskof spontaneous decomposition. The top stream from the first distillationcolumn is fed to a second distillation column and in this is separatedinto a top stream comprising propyne, possibly further low boilers andpossibly water and a bottom stream comprising pure 1,3-butadiene.

The stream fed to the work-up by distillation comprises predominantly1,3-butadiene and is therefore referred to as crude 1,3-butadienestream.

The composition of the crude 1,3-butadiene stream depends on thecomposition of the C₄ fraction which was fed to the extractivedistillation and generally comprises all the acetylenes, all the1,2-butadiene, from 30-70% of the cis-2-butene and at least 99% of the1,3-butadiene from the C₄ fraction.

For the present purposes, the hydrocarbons which have boiling pointslower than that of 1,3-butadiene are referred to as low boilers and thehydrocarbons which have boiling points higher than that of 1,3-butadieneare referred to as high boilers. A typical low boiler is propyne, andhigh boilers are predominantly hydrocarbons having a triple bond,hereinafter referred to as acetylenes, in particular 1-butyne(ethylacetylene) and butenyne (vinylacetylene).

The term “possibly” used in the present description in the context ofthe composition of streams obtained in the work-up by distillation meansthat the components qualified in this way may be present in therespective streams depending on the specific process conditions, inparticular depending on the composition of the C₄ fraction used, thesolvent used and/or auxiliaries used.

The separation of the acetylenes and 1,2-butadiene from the crude1,3-butadiene by distillation is a complicated distillation problembecause of their high reactivity and the small differences in therelative volatilities of the components making up the crude1,3-butadiene stream. However, it has surprisingly been found that theacetylenes and 1,2-butadiene can be separated off by distillation with ajustifiable energy consumption and at the same time a safe process canbe ensured when the acetylenes and 1,2-butadiene are taken off as bottomstream from a distillation column and are thus diluted with1,3-butadiene to outside the range in which there is a risk ofspontaneous decomposition. For this purpose, dilution of the bottomstream to below 30 mol % of acetylenes is generally sufficient.

In a preferred process variant, the crude 1,3-butadiene stream taken offfrom the extractive distillation column or a downstream desorptioncolumn is therefore subjected, in a distillation column, to fractionaldistillation which does not produce a sharp separation in respect of1,3-butadiene. Here, the acetylenes are taken off as a bottom streamwhich is diluted with 1,3-butadiene to outside the range in which thereis a risk of spontaneous decomposition. Otherwise, butadiene togetherwith propyne and possibly further low boilers is taken off from the topof the distillation column.

The top stream from the distillation column is preferably condensed in acondenser at the top of the column, part of the condensate is returnedas runback to the column and the remainder is passed to a seconddistillation column in which it is separated into a top streamcomprising propyne and possibly further low boilers and a bottom streamcomprising pure 1,3-butadiene.

In both of the above-described distillation columns, it is in principlepossible to use all separation-active internals customary for butadienedistillation. Owing to their lower fouling tendency, trays areparticularly useful.

For the present purposes, the term pure 1,3-butadiene refers to a streamhaving a 1,3-butadiene content of at least 99% by weight, preferably atleast 99.6% by weight, with the balance being impurities, in particular1,2-butadiene and cis-2-butene.

In a preferred process variant, the bottom stream from the firstdistillation column and the top stream from the second distillationcolumn are passed to a reactive distillation column in which aheterogeneously catalyzed selective hydrogenation of the hydrocarbonscontaining triple bonds to hydrocarbons containing double bonds iscarried out by means of hydrogen to give a top stream comprising1,3-butadiene, butenes, butenes and residual non-hydrogenatedhydrocarbons having triple bonds and a bottom stream comprising highboilers which is discharged.

In particular, vinylacetylene is selectively hydrogenated to the usefulproduct 1,3-butadiene.

The top stream from the reactive distillation column is preferablyrecycled to the extractive distillation column. However, it is alsopossible to take the stream from the top of the reactive distillationcolumn or a substream thereof from the plant and process it further, forexample in a cracker, or burn it.

The preferred process variant with selective hydrogenation of theacetylenes downstream of the extractive distillation is advantageousfrom a process engineering point of view, in particular in respect ofthe possible choices of catalyst, since the selective hydrogenation iscarried out in a process step in which virtually no selective solventremains in the reaction mixture. If, on the other hand, the selectivehydrogenation were to be carried out, as in known processes, in theextractive distillation column and thus in the presence of the selectivesolvent, the choice of catalyst would be restricted considerably by theselective solvent which can make the hydrogenation less selective. Incontrast, the selective hydrogenation downstream of the extractivedistillation is subject to no such restrictions in terms of the choiceof catalyst.

The present invention also provides an extractive distillation columnfor fractionating a C₄ fraction by extractive distillation using aselective solvent, wherein a dividing wall is installed in thelongitudinal direction in the extractive distillation column to form afirst region, a second region and a lower combined column region, with atop stream comprising the butanes being taken off from the first region,a top stream comprising the butenes being taken off from the secondregion and a stream comprising the hydrocarbons from the C₄ fractionwhich are more soluble in the selective solvent than are the butanes andthe butenes being taken off from the lower combined column region.

The invention is illustrated below with the aid of a drawing andexamples.

In the drawing:

FIG. 1 schematically shows an extractive distillation column EDK withdividing wall TW, and

FIG. 2 schematically shows a preferred embodiment of a plant forfractionating a C₄ fraction in an extractive distillation column withdownstream fractional distillation of the crude 1,3-butadiene streamfrom the extractive distillation column in two distillation columns anddownstream selective hydrogenation.

The extractive distillation column EDK schematically shown in FIG. 1 hasa dividing wall TW which is installed in the longitudinal direction ofthe column and continues through to the uppermost point of theextractive distillation column EDK and divides the column into a firstregion A, a second region B and a lower combined column region C. The C₄fraction C₄ is fed into the extractive distillation column EDK in itsregion A.

The plant depicted schematically in FIG. 2 shows an extractivedistillation column EDK for fractionating a C₄ fraction (C₄) which has adividing wall TW which is installed in the longitudinal direction in theupper part of the column and divides the extractive distillation columnEDK into a first region A, a second region B and a lower combined columnregion C. The C₄ fraction (C₄), which, as shown in the figure, canadvantageously be heated, in particular vaporized, in a heat exchangerby heat exchange with the selective solvent LM, is fed into theextractive distillation column EDK in the first region A of the column.A liquid stream of the selective solvent LM which, as shown in thefigure, is advantageously cooled by heat exchange with the C₄ fractionand subsequently in a condenser is introduced, in each case from thetop, into each of the two regions A and B. A top stream C₄H₁₀ comprisingthe butanes is taken off from the first region A of the extractivedistillation column EDK, condensed in a condenser K, part of thecondensate is returned as runback to the first region A of theextractive distillation column EDK and the remainder is taken off.Analagously, a top stream C₄H₈ comprising the butenes is taken off fromthe second region B of the extractive distillation column EDK, condensedin a condenser K, part of the condensate is returned as runback to thesecond region B and the remainder is taken off.

A stream C₄H₆ comprising the hydrocarbons which are more soluble in theselective solvent than are the butanes and the butenes, predominantly1,3-butadiene, is taken off from the lower combined column region C ofthe extractive distillation column EDK.

The stream C₄H₆ is preferably fed, as shown in FIG. 2, to a short sidecolumn S in which the stream C₄H₆ is distilled and a bottom streamcomprising the solvent is taken off and returned to the extractivedistillation column EDK. The short side column, whose use is notabsolutely necessary, thus serves to recover traces of solvent from thecrude 1,3-butadiene stream.

A stream comprising predominantly the selective solvent LM is taken offfrom the bottom of the extractive distillation column EDK. The heat inthe solvent is, depending on specific conditions related to the sitewhere the plant is located, in particular availability of coolants,integration into other plants or further processing chains, removed viavarious heat exchangers and the cooled solvent stream is finallyrecycled to the extractive distillation column EDK, in the upper regionthereof.

The side stream from the extractive distillation column EDK, namely thestream C₄H₆, for the present purposes referred to as crude 1,3-butadienestream, is fed to a first distillation column K I where it is separatedinto a top stream K I-K and a bottom stream K I-S. The top stream K I-Kis condensed in a condenser K at the top of the column, part of thecondensate is returned as runback to the column and the remainder istaken off and passed to a second distillation column K II. The bottomstream K I-S is taken off and fed to a reactive distillation column RDK.

In the second distillation column K II, the condensate from the firstdistillation column is fractionated to give a top stream K II-K which iscondensed in a condenser K, part of the condensate is returned asrunback to the column and the remainder is likewise passed to thereactive distillation column RDK. The bottom stream K II-S from thesecond distillation column K II is taken off as pure 1,3-butadienestream.

In the reactive distillation column RDK, the hydrocarbons containingtriple bonds are selectively hydrogenated to hydrocarbons containingdouble bonds by means of hydrogen in the presence of a heterogeneouscatalyst. A top stream RDK-K is taken off, condensed in a condenser K,part of the condensate is returned to the reactive distillation columnRKD and the remainder is preferably, as shown in the figure, recycled tothe extractive distillation column EDK.

The bottom stream from the reactive distillation column, namely streamRDK-S, which comprises predominantly high boilers, is discharged fromthe plant and is preferably burnt.

EXAMPLE Extractive Distillation

A C₄ fraction from a steam cracker having the composition reported belowin % by weight was fed to an extractive distillation column which wasconfigured as a dividing wall column having a dividing wall goingthrough to the uppermost point of the column and had a total of 80theoretical plates of which 25 were in the region of the dividing wall.The C₄ fraction was introduced into the first region denoted in thedrawing by A on the 68th theoretical plate, counting from the bottom.NMP containing 8.3% by weight of water was used as selective solvent.

Propene 0.02 propadiene 0.04 propyne 0.06 n-butane 5.74 i-butane 2.44n-butene 13.88 i-butene 25.63 t-2-butene 4.44 cis-2-butene 2.951,3-butadiene 43.81 1,2-butadiene 0.14 1-butyne 0.12 vinylacetylene 0.73

A top stream comprising predominantly butanes was taken off from thefirst region A of the extractive distillation column EDK; this had thefollowing composition in % by weight:

Propene 0.19 n-butane 62.02 i-butane 27.98 n-butene 6.63 i-butene 2.71trans-2-butene 0.24 H₂O 0.23

A top stream comprising predominantly butenes was taken off from theregion B of the extractive distillation column; this had the followingcomposition in % by weight:

Propadiene 0.07 n-butane 0.91 i-butane 0.10 n-butene 28.57 i-butene54.55 trans-2-butene 9.48 cis-2-butene 6.32

Compared to a known process in which butanes and butenes are separatedoff together in an extractive distillation column and are subsequentlyseparated in an additional apparatus, an energy saving of about 20% wasachieved.

EXAMPLE Work-Up of Crude 1,3-butadiene by Distillation

A crude 1,3-butadiene stream obtained by extractive distillation asdescribed in the above example from the C₄ fraction described there wasfed to a distillation column having 80 theoretical plates on the 25thplate, counted from the bottom. The crude 1,3-butadiene stream C₄H₆ hadthe following composition in % by weight:

Propyne 0.11 1,3-butadiene 98.58 1,2-butadiene 0.30 1-butyne 0.30vinylacetylene 0.56 water 0.15

In the distillation column K I, this stream C₄H₆ was separated into atop stream K I-K having the following composition in % by weight:

Propyne 0.11 1,3-butadiene 99.73 water 0.16and a bottom stream K I-S having the following composition in % byweight:

cis-2-butene 0.52 1,3-butadiene 40.0 1,2-butadiene 15.1 1-butyne 13.75vinylacetylene 29.17 3-methyl-1-butene 0.98 2-methyl-2-butene 0.48

The top stream K I-K from the first distillation column K I was dividedinto an offtake stream (1/7 of the top stream K I-K) and a runbackstream (6/7 of the top stream k I-K). The offtake stream was fed into asecond distillation column K II having 25 theoretical plates on the 14ththeoretical plate and fractionated to give a top stream K II-K havingthe following composition in % by weight:

Propyne 79.52 1,3-butadiene 20.0 and water  0.48and a bottom stream K II-S comprising pure 1,3-butadiene and having a1,3-butadiene content of 99.99%. The bottom stream K II-S was taken offas desired product.

The energy which had to be supplied from the outside in theabove-described fractionation according to the present invention of a C₄fraction in an extractive distillation column to give a streamcomprising the butanes, a stream comprising the butenes and a streamcomprising crude 1,3-butadiene with subsequent work-up of the crude1,3-butadiene stream by distillation was 15% lower than in the case of aknown process in which butanes and butenes are separated off in separateprocess stages, as described, for example, in the German patentapplication 102 19 375, and the acetylenes are separated off from thecrude 1,3-butadiene stream by extractive distillation using a selectivesolvent, as described, for example, in the German patent application 10105 660.

1. A continuous process for fractionating a C₄ fraction by extractivedistillation using a selective solvent in an extractive distillationcolumn having a dividing wall that extends in the longitudinal directionto an uppermost point of the extractive distillation column to form afirst region, a second region and a lower combined column region,wherein the process comprises: taking off from the first region a topstream comprising predominantly one or more butanes; taking off from thesecond region a top stream comprising predominantly one or more butenes;and taking off from the lower combined column region a stream comprisingone or more hydrocarbons from the C₄ fraction which are more soluble inthe selective solvent than are the butanes and the butenes.
 2. Theprocess according to claim 1, wherein the stream comprising thehydrocarbons from the C₄ fraction which are more soluble in theselective solvent than are the butanes and the butenes is taken off as aside stream from the lower combined column region and the selectivesolvent is taken off as a bottom stream from the extractive distillationcolumn.
 3. The process according to claim 1, wherein the streamcomprising the hydrocarbons from the C₄ fraction which are more solublein the selective solvent than are the butanes and the butenes is takenoff together with the selective solvent as a bottom stream from theextractive distillation column.
 4. The process according to claim 1,wherein the C₄ fraction is fed into the first region of the extractivedistillation column, the top stream comprising the butanes is taken offfrom the first region of the extractive distillation column and the topstream comprising the butenes is taken off from the second region of theextractive distillation column.
 5. The process according to claim 1,wherein two or more, thermally coupled columns are used in place of theextractive distillation column with dividing wall.
 6. The processaccording to claim 1, wherein the selective solvent comprises one ormore substances selected from the group consisting of dimethylformamide,acetonitrile, furfural, and N-methylpyrrolidone.
 7. The processaccording to claim 1, wherein 10-80 theoretical plates are located inthe region of the dividing wall of the extractive distillation column.8. The process according to claim 1, wherein a heterogeneously catalyzedselective hydrogenation of the hydrocarbons comprising triple bonds fromthe C₄ fraction to hydrocarbons comprising double bonds is additionallycarried out in the extractive distillation column.
 9. The processaccording to claim 1, wherein the stream comprising the hydrocarbonswhich are more soluble in the selective solvent than are the butanes andbutenes which is taken off from the extractive distillation column isfed to a first distillation column in which it is separated into a topstream comprising 1,3-butadiene, propyne, possibly further low boilersand possibly water, and a bottom stream comprising 1,3-butadiene,1,2-butadiene, acetylenes and possibly further high boilers, with theproportion of 1,3-butadiene in the bottom stream from the distillationcolumn being regulated so that it is sufficiently high to dilute theacetylenes to outside the range in which there is a risk of spontaneousdecomposition and the top stream from the first distillation column isfed to a second distillation column and in this is separated into a topstream comprising propyne, possibly further low boilers and possiblywater and a bottom stream comprising pure 1,3-butadiene.
 10. The processaccording to claim 9, wherein the bottom stream from the firstdistillation column and the top stream from the second distillationcolumn are passed to a reactive distillation column in which aheterogeneously catalyzed selective hydrogenation of the hydrocarbonscomprising triple bonds to hydrocarbons comprising double bonds iscarried out by means of hydrogen, with a partial conversion of theacetylenes, to give a top stream comprising 1,3-butadiene, butanes,butenes and non-hydrogenated hydrocarbons having triple bonds and abottom stream comprising high boilers which is discharged.
 11. Theprocess according to claim 1, further comprising processing the streamcomprising the butenes, isobutene, 1-butenes and 2-butenes in a reactivedistillation column to give a stream comprising predominantly isobuteneand a stream comprising predominantly 2-butenes, with 1-butene beinghydroisomerized to 2-butenes in the reactive distillation column and thestream comprising predominantly isobutene being taken off as a topstream from the reactive distillation column and the stream comprisingpredominantly 2-butenes being taken off as a bottom stream from thereactive distillation column.
 12. The process according to claim 1,further comprising subjecting the stream comprising the butenes to aselective etherification of the isobutene and fractionation to give astream comprising the isobutene ether and a stream comprising 1-buteneand 2-butenes and subsequently further processing the stream comprising1-butene and the 2-butenes by gas-phase isomerization of the 2-butenesto give a stream comprising predominantly 1-butene or byhydroisomerization of the 1-butene to give a stream comprisingpredominantly 2-butenes.
 13. The process according to claim 1, furthercomprising processing the stream comprising the butenes, isobutene,1-butene and 2-butenes, by skeletal isomerization of 1-butene and2-butenes to isobutene, to give a stream comprising predominantlyisobutene.
 14. The process according to claim 1, further comprisingprocessing the stream comprising the butenes, isobutene, 1-butene and2-butenes, by separating off isobutene and working it up by skeletalisomerization to give a stream comprising predominantly 1-butene and2-butenes.
 15. The process according to claim 1, further comprisingprocessing the stream comprising the butenes, isobutene, 1-butene and2-butenes, by separating off isobutene and processing it further byhydrogenation to give a stream which comprises predominantly isobutaneand is preferably fed to a cracker or by skeletal isomerization to givea stream comprising predominantly n-butane and dehydrogenation of thelatter to give a stream comprising predominantly 1-butene and 2-butenes.16. The process according to claim 1, further comprising processing thestream comprising the butenes, isobutene, 1-butene and 2-butenes, byselective dimerization of isobutene to the corresponding C₈-hydrocarbonsand subsequent fractional distillation to give a stream comprising1-butene and 2-butenes and a stream comprising the C₈-hydrocarbons. 17.The process according to claim 5, wherein two or three, thermallycoupled columns are used in place of the extractive distillation columnwith dividing wall.
 18. The process according to claim 6, wherein theselective solvent is N-methylpyrrolidone in an aqueous solution.
 19. Theprocess according to claim 7, wherein 25 theoretical plates are locatedin the region of the dividing wall of the extractive distillationcolumn.
 20. The process according to claim 1, wherein the content of theone or more butanes in the top stream comprising predominantly one ormore butanes is at least 60 wt. %, and the content of the one or morebutenes in the top stream comprising predominantly one or more butenesis at least 60 wt. %.
 21. The process according to claim 1, wherein thecontent of the one or more butanes in the top stream comprisingpredominantly one or more butanes is at least 80 wt. %, and the contentof the one or more butenes in the top stream comprising predominantlyone or more butenes is at least 80 wt. %.
 22. The process according toclaim 1, wherein the content of the one or more butanes in the topstream comprising predominantly one or more butanes is at least 95 wt.%, and the content of the one or more butenes in the top streamcomprising predominantly one or more butenes is at least 95 wt. %.